1. Field of the Invention
The invention relates to electronic communications and, more particularly, to a phase detector in the carrier-recovery mechanism of a differential-quadriphase-shift-keying (DQPSK) digital receiver.
2. Description of the Related Art
Quadriphase shift keying (QPSK) is a technique of phase modulating digital information onto a carrier signal. QPSK communications systems are generally known in the art. In these systems, a transmission unit uses a local reference oscillator to generate a carrier wave. This oscillator in the transmitter determines the frequency and phase of the unmodulated carrier wave. The transmitter encodes two bits of digital information on the carrier by shifting the carrier phase by a multiple of 90.degree. for an interval of time of length T. The four possible phase shifts, or symbols, that can be transmitted during this interval are determined by the four possible combinations of the two bits to be transmitted. One symbol (two bits of information) is transmitted during each time interval, so the rate of transfer of data in the system is two bits per symbol interval T. On the other end of this system, a receiving unit decodes the two transmitted bits by measuring the phase shift between the received signal and a local reference oscillator.
A drawback to this system is the requirement that the receiver must have a reference oscillator matched in phase to the transmitter oscillator; that is, the decoding is done by coherent detection. This requirement is relaxed in the technique of differential QPSK (DQPSK). In DQPSK, the transmitted data are differentially encoded, that is, they are represented by the difference in phase between successive symbols. In this technique, the receiver does not need the absolute phase of the transmitter oscillator to decode the transmitted symbols. Instead, the decoding of the symbols is by differentially coherent detection: the receiver measures the phase difference between two successive received symbols. This measurement yields a number with four possible values (0.degree., 90.degree., 180.degree., 270.degree.) that represents the two bits of transmitted data.
To demodulate the received signal, it is desirable for the DQPSK receiver to have a local oscillator that reconstructs the carrier wave. This oscillator in the receiver must match the frequency (although not necessarily the phase) of the DQPSK transmitter oscillator that generated the carrier wave. If the frequencies of the two oscillators are not matched, the receiver cannot efficiently demodulate the transmitted data. The receiver oscillator can be built so that its natural frequency is close to that of the transmitter, but due to variations in manufacturing and differences in operating environments, there will be drifts between the two oscillators. To compensate for such offsets in frequency between the carrier wave and the receiver oscillator, the receiver oscillator can be locked to the carrier wave by a phase-locked loop (PLL). Such a carrier-recovery mechanism can serve to tie the frequency of the receiver oscillator to the frequency of the transmitter oscillator.
This carrier-recovery mechanism must be able to ignore variations in the carrier phase that are due to the information encoded into the carrier. That is, changes in the phase of the carrier by multiples of 90.degree. must not be interpreted as a drift in the receiver oscillator's frequency. There are several established methods of making the PLL in the carrier-recovery mechanism insensitive to the data-bearing 90.degree. shifts.
One method involves generating the fourth power of the received signal. This new signal has a high-frequency component at four times the carrier frequency. The phase of this high-frequency component is coherent with the carrier and independent of the phase shifts representing the encoded data. This high-frequency component can thus serve as a good reference for locking the receiver oscillator. Various Costas-loop circuits can also be used to lock the oscillator; these circuits have characteristics similar to the fourth-power lock.
A second method for locking the receiver oscillator to the unmodulated carrier involves examining two components of the received signal: one that is in phase with the receiver oscillator (I), and one that is 90.degree. out of phase with the receiver oscillator (Q). The I component has a maximum magnitude when the receiver oscillator and the received signal are in phase or 180.degree. out of phase, and a zero magnitude when they are 90.degree. or 270.degree. out of phase. The Q component has the opposite behavior: its magnitude is zero when the I component's magnitude is greatest, and a maximum when the I component's magnitude is zero. The I and Q components have equal magnitudes when the receiver and transmitter oscillators differ in phase by .+-.45.degree. or .+-.135.degree.. Since this condition occurs at four different phase shifts 90.degree. apart, it is insensitive to jumps in the received signal's phase by 90.degree.. Therefore, the balance between the I and Q components can also provide a good reference for the PLL in the carrier-recovery mechanism. By adjusting the receiver oscillator so that the magnitudes of the I and Q components remain equal, the PLL can keep the receiver oscillator's phase locked to a constant shift (of 45.degree., -45.degree., 135.degree., or -135.degree.) from the transmitter oscillator's phase. Two oscillators thus locked to within a constant phase shift are consequently also locked in frequency, so the receiver oscillator can provide the appropriate reference for demodulating the transmitted data.
An elementary method of keeping the I and Q magnitudes equal is to multiply them together and use a PLL to maximize the resulting product. This method, however, requires use of a multiplication block, which adds complexity to the system. The present invention overcomes this problem of the prior technology by providing a phase detector that measures the I and Q components and directly constructs a phase-error signal that indicates an imbalance between them using simple arithmetic operations. The present invention thus provides a significant improvement and advance in the art and technology of carrier recovery in DQPSK receivers.